It has been recognized that there is a need to monitor the structural health of various types of vehicles, machines and other types of equipment. To name a few examples, structural health monitoring is desirable in aircraft, critical pressure vessels used in the energy generating industry, oil tanks and pipelines, and the like.
A variety of techniques have been utilized to perform nondestructive evaluation of structures. For example, ultrasonic transducers have been used in portable devices placed in communication with the structure of interest. The crystals propagate ultrasonic waves into the material, which are then detected. The detected signals can be electronically processed to indicate anomalies.
An embedded nondestructive evaluation system for structures is described in U.S. Pat. No. 6,006,163 to Lichtenwalner. This patent describes an active damage interrogation system having an array of piezoelectric transducers attached to or embedded within a structure for both actuation and sensing. After interrogation, the transducer signals are digitized and the transfer function of an actuator/sensor pair is computed. The computed transfer function is compared against a baseline transfer function for the respective pair. The transducers are actuated by broadband excitation over a frequency range of 0 to 100 kHz.